1. Field of the Invention
The embodiment relates to a fingerprint sensor.
2. Description of Related Art
Recently, a fingerprint sensor, which detects the fingerprint of a person, has been extensively used even to determine the on/off of power and the release of a sleep mode in an electronic device, as well as a door lock, to which the fingerprint sensor has been conventionally widely applied.
Fingerprint sensors may be classified into an ultrasonic fingerprint sensor, an infrared fingerprint sensor, and a capacitive fingerprint sensor according to the operating principles thereof. For example, the ultrasonic fingerprint sensor employs a scheme of detecting a fingerprint by measuring the differences between acoustic impedances of valleys and ridges measured using relevant piezoelectric sensors, which are sources of ultrasonic waves, when ultrasonic signals having predetermined frequencies emitted from the piezoelectric sensors are reflected from the valleys and the ridges of the fingerprint. In particular, according to the advantages of the ultrasonic fingerprint sensor, in addition to a function of simply recognizing a fingerprint, the flow of blood in a finger can be detected by detecting a Doppler effect of reflected waves of the ultrasonic waves generated in the form of a pulse. Accordingly, even a fake fingerprint can be determined using the flow of the blood in the finger.
In the fingerprint sensor, a piezoelectric sensor is provided on a substrate, and electrodes are provided on both surfaces of the piezoelectric sensor to recognize the fingerprint as the ultrasonic waves are generated. However, when the piezoelectric sensor is directly provided on the substrate, the vibration of the piezoelectric sensor is restricted due to the bond between the piezoelectric sensor and the substrate. Accordingly, the characteristic of transmitting or receiving the ultrasonic wave may be degraded.
In addition, a resonance frequency band characteristic, and an ultrasonic wave generation characteristic are changed according to the characteristics of a piezoelectric sensor material. Accordingly, the resonance frequency band characteristic and the ultrasonic wave generation characteristic may be changed according to materials selected for the piezoelectric sensor.
In addition, when the piezoelectric material is provided, the haze value of the piezoelectric material is increased, so that the piezoelectric sensor may be viewed from the outside.
Further, the ultrasonic wave generated from the piezoelectric sensor toward the cover substrate may be more smoothly transmitted as the difference between the acoustic impedances of the piezoelectric sensor and the cover substrate is reduced. To this end, a buffer layer can be provided to reduce the difference between the acoustic impedances of the piezoelectric sensor and the cover substrate.
In this case, when the buffer layer is provided in a multi layer structure instead of a single layer structure, the process efficiency may be degraded, and the thickness of the buffer layer may be increased.
Accordingly, there is required a fingerprint sensor having a novel structure capable of solving the above problems.